Microfield donor with continuously reversing microfields

ABSTRACT

Electrostatic latent image development method is carried out by an apparatus employing a microfield donor member wherein the microfields are continuously reversed to nullify the effect of charge buildup at dielectric interfaces and within the bulk of the dielectrics.

United States Patent 1 Maksymiak et al.

[451 Sept. 18, 1973 MICROFIELD DONOR WITH CONTINUOUSLY REVERSING MICROFIELDS [75] Inventors: John Maksymiak, Penfield; Richard B. Lewis, Williamson, both of N.Y.

[73] Assigneef Xerox Corporation, Stamford, Conn.

[22] Filed: Mar. 4, 1971 [21] Appl. No.: 121,090

[52] US. Cl 118/637, 96/l SD, ll7/l7.5,

355/3 [51] Int. Cl. G03g 13/08, G03g 15/08 [58] Field of Search ll7/l7.5; 118/637;

96/l R, 1 SD; 355/3, 17

[56] References Cited UNITED STATES PATENTS 8/1965 Hope et al. 1l7/l7.5

3,216,844 ll/l965 King ll7/l7,5 3,332,396 7/l967 Gundlach ll7/l7.5 3,l52,0l2 l0/l964 Schaffert ll7/l7.5

Primary Examiner-William D. Martin Assistant Examiner--M. Sofocleous Attorney-James J. Ralabate, Donald F. Daley and James P. OSullivan [57] ABSTRACT 4 Claims, 4 Drawing Figures Pmmcnwww A 3.759.222 88511 of FIG.

INVENTO JOHN MAKSY fiIAK- RICHARD B. LEWIS ym www ATTORNEY MICROFIELD DONOR WITH CONTINUOUSLY REVERSING MICROFIELDS BACKGROUND OF THE INVENTION,

This method and apparatus relates to development of electrostatic latent images and more particularly to more effective use of a microfield donor member for long-term use in presenting toner particles to a latent image for development thereof.

In conventional xerography, a photoconductive surface is charged and then exposed to a light pattern of the information to be recorded or reproduced, thereby forming an electrostatic latent image on the photoconductive surface. Toner particles, which may be finely divided, pigmented, resinous material, are presented to the latent image where they are attracted to the photoconductive surface. The toner image can be fixed and made permanent on the photoconductive surface or it can be transferred to another surface where it is fixed.

One known method of developing electrostatic latent images is by a process called transfer development. Transfer development broadly involves bringing a layer of toner to an imaged photoconductor where toner particles will be transferred from the layer to the imaged areas. The term transfer development is generic to development techniques where (1) the toner layer is out of contact with the imaged photoconductor and the toner particles must traverse an air gap to effect development, (2) the toner layer is brought into rolling contact with the imaged photoconductor to effect development, and (3) the toner layer is brought into contact with the imaged photoconductor and skidded across the imaged surface to effect development. Transfer development has also come to be known as touchdown development".

In a typical transfer development system, a cylindrical or endless donor member is rotated so that its surface can be presented to the moving surface of a photoconductive drum bearing an electrostatic latent image thereon. Positioned about the periphery of the donor member are processing stations which may include some or all of the following: a donor loading station, at which toner is made to adhere to the donor member surface; an agglomerate removal station at which toner agglomerates are removed from the retained toner layer; a charging station at which a uniform charge is placed on the retained toner; a cleanup station at which the retained toner layer is converted into one of uniform-thickness and any remaining agglomerates are removed from the retained toner layer; a development station at which the retained toner is presented to the imaged photoconductor for image development; and a cleaning station at which a neutralizing charge is placed upon the residual toner particles and at which a cleaning member removes residual toner from the peripheral surface of the donor. In this manner, a more or less continuous development process is carried out.

Among the donor members employed in the process and apparatus described is one embodying the principles described in U. S. Pat. No. 3,203,394. Such a donor includes, an electrically conductive support member in the form of a cylinder, a thin electrically insulating layer overlying the support member, and'a continuous, electrically conductive screen pattern overlying the insulating layer. A protective dielectric layer may be flush with or just cover the conductive screen pattern. A potential difference between the conductive screen pattern and the conductive substrate creates fringe fields or microfields which extend into the air region beyond the outer surface of the donor member. It is the extended portion of these microfields which attract and hold the toner particles to the surface of the donor member for subsequent presentation to the imaged photoconductor.

It has been determined, however, that after a short period of time the microfields become diminished or destroyed and toner particles are no longer effectively adhered to the surface of the donor member.

The art of latent electrostatic image development and in particular transfer development would be significantly advanced if the microfield attractive force could be maintained undiminished during repeated development sequences.

Accordingly, it is an object of the invention to improve apparatus for development of electrostatic latent images. 4

it is a further object to improve donor member apparatus for developing electrostatic latent images.

Still another object of the invention is to improve upon transfer development processes.

Yet another object is to improve upon microfield donor loading techniques by providing for continuously effective microfields.

SUMMARY OF THE INVENTION Since only those microfields extending beyond the surface of the donor member are effective to attract toner particles, there must always be microfield lines penetrating the dielectric interface or interfaces. During use of the microfield donor, if the voltage source is from DC. and since the dielectric regions will always have some conductivity, charges migrating within the structure will eventually modify the microfields, diminishing or destroying the effective fields in the region just beyond the surface of the microfield donor. For example, if the conductivity of the outer dielectric region just beyond the inner dielectric film separating the screen electrode from the conductive substrate, i.e., the air dielectric or a second dielectric film, much exceeds that of the inner dielectric film, the interface between these dielectrics tends to approach or equal the potential of the voltage source. Asa result no fine structured microfield will exist beyond this interface and effective toner attraction will be lost. The time it takes for this condition to develop can be termed the equilibrium time. Details of the equilibrium field structure depend upon the relative conductivities of the dielectrics involved. Even if the inner dielectric and the outer dielectric were of the same material, conditions such as humidity or toner contamination would make the layers sufficiently electrically different that an equilibrium condition would rapidly occur, destroying the effectiveness of the microfields.

it has been discovered that these microfield nulling effects may be controlled by periodically reversing the direction of the microfields involved by switching the polarity of the voltage source. Charges distributed or accrued within the dielectric and at the dielectric interfaces in the structure are then periodically removed if the time between polarity reversals is appreciably smaller than the equilibrium time.

Accordingly, the present invention is directed to a developing method for electrostatic latent images formed on the surface of an image-retaining member.

After formation of an electrostatic latent image on an image retaining surface, at least a portion of the surface of a donor member is sequentially brought to a plurality of treating stations. The treating stations include: a toner loading station, having a supply of toner particles at which a layer of toner particles becomes adhered to the surface of the donor member; and a development station, at which the toner particles carried by the donor member are presented in developing relation to the latent image. The donor member comprises a dielectric layer separating a conductive screen pattern electrode from a conductive continuous film electrode. As the surface of the donor member is brought to the toner loading station, one electrode is maintainted at a reference potential and the other is maintained at a sufficiently different potential so as to create microfields extending into the region beyond the surface of the donor member to attract to and hold toner particles on the surface of the donor member carrying the screen electrode. After the donor has been loaded with toner particles, a reversal in the direction of the microfields.

tinuous film electrode. As each screen pattern electrode region of the donor member is brought to the toner loading station, a potential difference is maintained between the screen electrodes and the film electrodes sufficient to create microfields extending into the region beyond the surface-of donor member to attract to and hold toner particles on the screen electrode regions of the donor member. After each screen electrode region has been loaded with toner particles, a reversal in the direction of the microfields is effected between each screen electrode and the film electrode for a sufficient time and at a sufficient potential to remove charges from the dielectric layer accrued during the time of toner loading Thereafter, the fields are sequentially returned to their former loading. direction prior to again bringing each screen pattern electrode region of the donor member to the toner loading station.

In another modification of the foregoing process, the donor member comprises an endless dielectric layer separating a continuous, electrically conductive screen pattern electrode from a plurality of mutually electrically isolated conductive film electrodes. As each film electrode region of the donor member is brought to the toner loading station, a potential difference is maintained between each film electrode and the screen electrode sufficient to create microfields extending into the region beyond the surface of the donor member to attract to and hold toner particles on the surface of the donor member carrying the screen electrode. After each film electrode region of the donor has been loaded with toner, a field reversal in the direction of the microfields between each film electrode and the screen electrode is sequentially effected for a sufficient time and at a sufficient potential to remove charges from the dielectric layer accrued during the time period of toner loading. Thereafter the fields are sequentially returned to their former direction prior to again bringing each film electrode region of the donor member to the toner loading station.

it is also contemplated to employ both a plurality of mutually, electrically isolated conductive screen pattern electrodes and a plurality of mutually, electrically isolated conductive film electrodes.

The invention is also directed to an apparatus for developing an electrostatic latent image formed on the surface of an image-retaining member. The apparatus includes a means for developing the latent image, said means including: (a) a microfield donor member adapted to transport toner particles to the latent image, said donor member comprising, a dielectric layer separating a conductive screen pattern electrode from a conductive continuous film electrode: (b) means to transport the donor member past a plurality of treating stations, said treating stations including: (1) a toner loading station including a supply of toner particles at which toner particles are contacted and a layer of toner particles retained by the donor member in response to microfields setup between the screen pattern electrode and the film electrode: and (2) a developing station at which the layer of retained toner particles is presented in developing relation to an electrostatic latent image on an image-retaining member, and (0) means adapted to maintain the potential difference between the electrodes sufficient to create microfields extending into the region beyond the surface of the donor member and of a magnitude sufficient to attract to and hold toner particles on the surface of the donor member carrying the screen electrode, the means being further adapted to effect a field reversal of said microfields at a time after toner loading of the donor, said reversal being effective to remove charges from the dielectric accrued during the time period of toner loading, said means being further adapted after charge removal, to return the microfields to their former directions.

In a preferred form of the present invention the apparatus includes a microfield donor member adapted to transport toner particles to the latent image comprising an endless dielectric layer separating a plurality of mutually electrically isolated conductive screen pattern electrodes from a conductive continuous film electrode. In this embodiment a means is employed which is adapted to maintain a potential difference between each screen electrode and the film electrode sufficient to create microfields extending into the region beyond the surface of the donor member and of a magnitude sufficient to attract to and hold toner particles on the surface of the donor member carrying the screen electrodes. This means is also adapted to sequentially effect a field reversal of the microfields between each screen electrode and the film electrode at a time after each screen electrode region has been loaded with toner particles. The reversal is effective to remove charges from the dielectric accrued during the time period of toner loading. This means is further adapted, after charge removal, to sequentially return said microfields to their former direction.

In another preferred embodiment of the present invention the apparatus includes a microfield donor member comprising an endless dielectric layer separating a continuous, electrically conductive screen pattern electrode from a plurality of mutually electrically isolated conductive film electrodes. With this type of donor member, a means is employed which is adapted -to sequentially maintain a potential difference between the screen electrode and each of the film electrodes sufficient to create microfields extending into the region beyond the surface of the donor member and of a magnitude sufficient to attract to and hold toner particles on the surface of the donor member carrying the screen electrode. This means is also adapted to sequentially effect a field reversal of the microfields between each film electrode and the screen electrode at a time after each film electrode region of the donor has been loaded with toner. The reversal is effective to remove charges from the dielectric accrued during the time period of toner loading. This means is further adapted, after charge removal, to sequentially return said microfields to their former direction.

The donor member may also have both a plurality of mutually electrically isolated screen pattern electrodes and a plurality of mutually electrically isolated film electrodes, separated by an endless dielectric layer. This will permit individual control of the potential difference between the electrodes at a plurality of regions about the periphery of the donor.

Other processing stations may be added to the arrangement of the present invention. For example, a toner agglomerate removal station may be located adjacent the periphery of the donor member at a point between the toner loading station and the development station, a uniform charging station may be located between the toner loading station and the development station and a residual or ghost image removal station may belocated at a point beyond the development station. I

BRIEF DESCRIPTION OF THE DRAWING For a better understanding of the drawing as well as other objects and further features thereof, reference is made tothe accompanying drawing, wherein:

FIG. 1 is a sectional view of xerographic apparatus in accordance with the present invention;

FIG. 2 is an isometric view of one section of a preferred microfield donor in accordance with the present invention;

FIG. 3 is an isometric view of one section of another preferred microfield donor in accordance with the present invention; and 1 FIG. 4 is an isometric view of one section of still another preferred microfield donor in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION The present invention is a transfer development system and method in which toner particles are applied to an electrostatic latent image on a photoconductive plate to develop the image. Although the system and method is described herein as part of a xerographic copier, it can be utilized in conjunction with any reproduction system wherein a latent image is to be developed by applying toner thereto.

Referring to FIG. 1, there is shown a xerographic reproduction apparatus utilizing the concept of the present invention. In this apparatus a xerographic plate is in the form of a drum which passes through stations A-E in the direction shown by the arrow. The drum has a suitable photosensitive surface,such as one including selenium overlying a layer of conductive material, on which a latent electrostatic image can be formed. The various stations about the periphery of the drum which carry out the reproduction process are: charging station A, exposing station B, developing station C, transfer station D, and cleaning station E. Stations A, B, D, and E represent more or less conventional means for carrying out their respective functions. Apart from their association with the novel arrangement to be described with respect to stations C they form no part of the present invention.

At station A, a suitable charging means 112, e.g., a corotron, places a uniform electrostatic charge on the photoconductive material. As the drum rotates, a light pattern, via a suitable exposing apparatus 14, e.g., a projector,is exposed onto the charged surface of drum 10. The latent image thereby formed on the surface of the drum is developed or made visible by the application by a finely divided pigmented, resinous powder called toner, at developing station C, which is described in greater detail below. After the drum is developed at station C, it passes through transfer station D comprising a copy sheet 16, corona charging device 18 and fusing device 20. Following transfer and fixing of the developed image to the copy sheet, the drum rotates through cleaning station E, comprising cleaning device 22, e.g., a rotating brush, at which residual toner is removed.

At developing station C, the apparatus includes a donor member 24 (more particularly described below) rotatably mounted adjacent a toner reservoir 26, containing a supply of toner particles 28. The donor member 24 is positioned so that a portion'of its periphery comes into contact with toner particles 28. The donor member is also located so as to provide a small gap between the surface of drum l0 and the outer surface of a toner layer carried by donor roll 24. As toner particles are presented to the electrostatic imaged regions of drum 10, the particles traverse this small gap thereby developing the latent image.

Asindicated above, the microfield nulling effects, due to charge accrual at the dielectric interface or in the dielectric, can be controlled by periodically reversing the direction of the microfields involved by switcha ing the polarity of the voltage source. Several means of accomplishing this are illustrated in FIGS. 2, 3 and d. The 'microfield donor M as illustrated in FIG. 2 shows one segment of a cylindrical donor member. The donor member shown consists of an endless dielectric layer 36 separating a continuous conductive screen pattern electrode 3% from a plurality of mutually electrically isolated conductive film electrodes 40. The screen pattern electrode can be constructed of wires or it may be formed of an etched metal film yielding open dielectric areas. Copper is a convenient metal and the dielectric islands can be from 0.01 mm to 0.08 mm in area. The screen pattern electrode 30 is held at some reference potential, for example, at ground potential, as shown at 42. By "film electrode" is meant usually a continuous solid metal layer. The thickness of this layer is not critical. It may be massive in comparison to the screen electrode. The electrically isolated conductive film electrodes d0 make contact with commutator segments 30 and 32 via lead and brush arrangements dd. Commutator segments 30 and 32 are connected to equal but opposite-in-sign voltage sources M and 48. With this arrangement the microfield donor, during rotation, will pass through two opposite electrical zones, defined by the imaginary line M as shown in FIG. I.

As microfield donor 24a revolves in the direction shown by the arrow in FIG. 1, and as a point on the micro field donor approaches toner tray 26, the voltage potential between one film electrode 40 and the screen electrode 38 will be placed at some predetermined level, e.g., 300 volts. This potential difference will be maintained at this segment while it is being transported through the toner tray and just past development region C. After all of this segment of microfield donor 24a passes just beyond development region C, the voltage potential is switched via the commutator to an opposite potential of predetermined level, e.g., +300 volts. This segment then remains at this potential until the segment traverses around to a point where it is again ready to enter into the toner tray 26, where it is again reversed in polarity to a potential of -300 volts. In this manner, charges accrued in the dielectric or at the dielectric interfaces are effectively removed by reason of the reversal of the direction of the microfields.

In the microfield donor shown in FIG. 3, donor 24b is essentially the opposite construction of that shown in FIG. 2. Continuous dielectric film 36 separates a plurality of mutually electrically isolated conductive screen pattern electrodes 50 from a conductive continuous film electrode 52. Conductive film 52 is held at a convenient reference potential, for example ground potential, as shown at 54. The individual electrically isolated conductive screen pattern electrodes 50 make contact with commutator sections 30 and 32 via lead and brush arrangements 44. The lead portions of this arrangement pass through dielectric layer 36 and insulated regions 56 of the conductive film electrodes 52. Toner loading is accomplished in the same manner as described with respect to FIG. 2. As donor 24b is rotated and one isolated screen electrode region 50 approaches toner tray 26, the voltage between electrode 52 and one of the screen electrodes 50 is maintained at say 300 volts. This potential is maintained during the period of time that this segment of the microfield donor is passing through the tray 26 and toner 28 and through development region C. After this segment passes just beyond development region C the potential is reversed to say +300 volts. This reversal causes a change in the direction of the microfields and it is maintained in this condition until it once again approaches the region just before entry into toner tray 26. At this point once again the field is reversed via commutator section 30'where again the potential across the electrodes is returned to -300 volts.

In the microfield donor segment 240, shown in FIG. 4, a third variation on FIGS. 2 and 3 is showmln this I manification, the microfield donor is constructed of a continuous dielectric layer 36 which separates a plurality of mutually electrically isolated conductive screen pattern electrodes 50 from a plurality of mutually electrically isolated conductive film electrodes 52. This microfield donor operates in the same manner as that illustrated in FIGS. 2 and 3. Each of the electrodes 50 are kept at a separate reference potential, for example ground potential, and the electrodes 52 obtain their potential via commutator sections 30 and 32, which respectively impart a positive potential during approximately one-half of the donor revolution and a negative potential during the other half.

For purposes of explanation the drawing shows imaginary line 34 defining two hemicircular zones through which the donor member passes. It is to be understood,

however, that the potential on a given segment of the donor need only be maintained while that segment is passing through the toner loading station. After leaving this station, and before arriving at the development station, the polarity of the field can be reversed. The conditions of reversal are dependent upon a potential-time relationship. For example, if the potential during loading is 300 volts, then the reversal can be +300 volts for the same period of time but no longer or a reverse nulling effect will come into play. This means that the commutator will have to be designed to hold this particular segment of the donor at a no-charge condition after a proper time period of reversal has been completed and there-after again bring this segment to the appropriate potential just prior to again entering the toner loading station.

Alternatively, if the loading potential is -300 volts, the reversal potential can be at a lower level e.g. +l50 volts, but for a longer period of time, or at a higher potential, e.g., +400 volts, for a shorter period of time. Also, the reversal period may be split into two or more periods to vary or suspend the potential. For example, to minimize the toner holding force at the development station, a no-field condition can be caused to exist in the donor segments as they pass this region.

It is to be understood that the invention is not limited to effecting field reversal by the particular means shown in FIGS. 2-4. Any means of causing a field reversal in order to remove the accrued charge is contemplated by the technique described. By the present invention, the nulling effect of the accrued charge is effectively. eliminated and the microfield donor can be used without interruption for extremely long periods of time or indefinately.

It is to be understood that while for purposes of illustration the donor member has been described basically as a cylinder, it may be an endless belt adapted to deliver toner from the toner source to the development region. For certain purposes a flat plate type of donor member may be utilized instead of an endless version.

Convention drive means, e.g., motors, belts, etc., are employed to drive the several movable members all in a manner well within the skill of the art.

Since many changes can be made in the above construction and many apparently widely difierent embodiments of this invention can be made without departing from the scope thereof, it is intended that all matter contained in the drawing and specification should be interpreted illustratively and not in a limited sense.

What is claimed is:

1. An apparatus for developing an electrostatic latent image formed on the surface of an image-retaining member, said apparatus comprising: means for developing said Iatent image, said means including:

a. a microfield doner member adated to transport toner particles on the outer face thereof to said latent image comprising, a dielectric layer sandwiched between and separating first conductive electrode means from second conductive film electrode means with said first electrode means being on the outer face of said dielectric layer and said film electrode means being on the inner face of said dielectric layer; said first electrode means being arranged in a pattern to set up microfields with said film electrode means for attracting said toner particles to said donor member;

. means adapted to maintain a potential difference between said electrodes sufficient to create voltage induced microfields extending into the-region beyond the surface of said donor member and of a magnitude sufficient to attract to and hold toner particles on the surface of said donor member carrying said electrode means, said last defined means being further adapted to effect a voltage induced field reversal of said microfields at a time after toner loading of the donor, said reversal being effective to remove charges from the dielectric accrued during the time period of toner loading, said last defined means being further adapted, after charge removal, to return the microfields to their former direction.

2. An apparatus for developing an electrostatic latent image formed on the surface of an image-retaining member, said apparatus comprising: means for developing said latent image, said means including:.

a microfie'ld donor member adapted to transport toner particles on the outer face thereof to said latent image comprising an endless dielectric layer sandwiched between and separating a plurality of mutually electrically isolated conductive screen pattern electrodes from a conductive continuous film electrode with said screen electrodes being on the outer face of said dielectric layer for attracting said toner particles to said donor member and said film electrode being on the inner face of said dielectric layer;

. means to transport the surface of said donor memmeans adapted to maintain a potential difference between each screen electrode and-said film electrode as each screen electrode region is brought to said loading station, said potential difference being sufficient to create voltage induced microfields extending into the region beyond the surface of said donor member and of a magnitude sufficient to attract to and hold toner particles on the surface of said donor member carrying said screen electrodes, said means being further adapted, after each screen electrode region is loaded with toner, to effect a voltage induced field reversal of said microfields,

said reversal being effective to' remove charges from said dielectric accrued during the time period of toner loading, said means being further adapted, after charge removal, to sequentially return the microfields to their former direction.

3. An apparatus for developing an electrostatic latent image formed on the surface of an image-retaining member, said apparatus comprising: means for developing said latent image, said means including:

LII

a. a microfield donor member adapted to transport toner particles on the outer face thereof to said latent image comprising an endless dielectric layer sandwiched between and separating a continuous, electrically conductive, screen pattern electrode from a plurality of mutually electrically isolated conductive film electrodes with said screen electrode being on the outer face of said dielectric layer for attracting said toner particles to said donor member and said film electrodes being on the inner face of said dielectric layer;

. means to transport the surface of said donor member past a plurality of treating stations, said treating stations including:

l. a toner loading station including a supply of toner particles at which toner particles are contacted by and a layer of toner particles retained by said donor member in response to microfields set up between said screen pattern electrode and said film electrodes; and

2. a developing station at which said layer of toner particles is presented in developing relation to an electrostatic latent image on an image-retaining member; and

. means adapted to maintain a potential difference between said screen electrode and each of said film electrodes as each film electrode region is brought to said loading station, said potential difference being sufficient to create voltage induced microfields extending into the region beyond the surface of said donor member and of a'magnitude sufficient to attract to and hold toner particles on the surface of said donor member carrying said screen electrode, said means being further adapted, after each film electrode region is loaded with toner, to effect a voltage induced field reversal of said microfields, said reversal being effective to remove charges from said dielectric accrued during the time period of toner loading, said means being further adapted, after charge removal, to sequentially return the microfields to their former direction.

4. An apparatus for developing an electrostatic latent image formed on the surface of an image-retaining member, said apparatus comprising: means for developing said latent image, said means including:

a. a microfield donor member adapted to transport toner particles on the outer face thereof to said latent image comprising an endless dielectric layer sandwiched between and separating a plurality of mutually electrically isolated conductive screen pattern electrodes from a plurality of mutually electrically isolated conductive film electrodes with said screen electrodes being on the outer face of said dielectric layer for attracting said toner particles to said donor member and said film electrodes being on the inner face of said dielectirc layer;

b. means to transport the surface of said donor member past a plurality of treating stations, said treating stations including:

1. a toner loading station including a supply of toner particles at which toner particles are contacted by and a layer of toner particles retained by said donor member in response to microfields set up between said screen pattern electrodes and said film electrodes; and

2. a developing station at which said layer of toner particles is presented in developing relation to an electrostatic latent image on an image-retaining member; and

c. means adapted to maintain a potential difference between said screen electrodes and said film electrodes as each screen electrode region is brought to 12 said loading station, said potential being sufficient to create voltage induced microfields extending into the region beyond the surface of said donor member and of a magnitude sufficient to attract to and hold toner particles on the surface of said donor member carrying said screen electrodes, said means being further adapted, after each screen electrode region is loaded with toner, to effect a voltage induced field reversal of said microfields, said reversal being effective to remove charges from said dielectric layer accrued during the time period of toner loading, said means being further adapted, after charge removal, to sequentially return the microfields to their former direction. a: a: 

1. An apparatus for developing an electrostatic latent image formed on the surface of an image-retaining member, said apparatus comprising: means for developing said latent image, said means including: a. a microfield doner member adated to transport toner particles on the outer face thereof to said latent image comprising, a dielectric layer sandwiched between and separating first conductive electrode means from second conductive film electrode means with said first electrode means being on the outer face of said dielectric layer and said film electrode means being on the inner face of said dielectric layer; said first electrode means being arranged in a pattern to set up microfields with said film electrode means for attracting said toner particles to said donor member; b. means to transport said donor member past a plurality of treating stations, said treating stations including:
 1. a toner loading station including a supply of toner particles at which toner particles are contacted by and a layer of toner particles retained by said donor member in response to microfields set up between said electrode means and said film electrode; and
 2. a developing station at which said layer of toner particles is presented in developing relation to an electrostatic latent image on an image-retaining member, and c. means adapted to maintain a potential difference between said electrodes sufficient to create voltage induced microfields extending into the region beyond the surface of said donor member and of a magnitude sufficient to attract to and hold toner particles on the surface of said donor member carrying said electrode means, said last defined means being further adapted to effect a voltage induced field reversal of said microfields at a time after toner loading of the donor, said reversal being effective to remove charges from the dielectric accrued during the time period of toner loading, said last defined means being further adapted, after charge removal, to return the microfields to their former direction.
 2. a developing station at which said layer of toner particles is presented in developing relation to an electrostatic latent image on an image-retaining member, and c. means adapted to maintain a potential difference between said electrodes sufficient to create voltage induced microfields extending into the region beyond the surface of said donor member and of a magnitude sufficient to attract to and hold toner particles on the surface of said donor member carrying said electrode means, said last defined means being further adapted to effect a voltage induced field reversal of said microfields at a time after toner loading of the donor, said reversal being effective to remove charges from the dielectric accrued during the time period of toner loading, said last defined means being further adapted, after charge removal, to return the microfields to their former direction.
 2. An apparatus for developing an electrostatic latent image formed on the surface of an image-retaining member, said apparatus comprising: means for developing said latent image, said means including: a. a microfield donor member adapted to transport toner particles on the outer face thereof to said latent image comprising an endless dielectric layer sandwiched between and separating a plurality of mutually electrically isolated conductive screen pattern electrodes from a conductive continuous film electrode with said screen electrodes being on the outer face of said dielectric layer for attracting said toner particles to said donor member and said film electrode being on the inner face of said dielectric layer; b. means to transport the surface of said donor member past a plurality of treating stations, said treating stations including:
 2. a developing station at which said layer of toner particles is presented in developing relation to an electrostatic latent image on an image-retaining member; and c. means adapted to maintain a potential difference between each screen electrode and said film electrode as each screen electrode region is brought to said loading station, said potential difference being sufficient to create voltage induced microfields extending into the region beyond the surface of said donor member and of a magnitude sufficient to attract to and hold toner particles on the surface of said donor member carrying said screen electrodes, said means being further adapted, after each screen electrode region is loaded with tOner, to effect a voltage induced field reversal of said microfields, said reversal being effective to remove charges from said dielectric accrued during the time period of toner loading, said means being further adapted, after charge removal, to sequentially return the microfields to their former direction.
 2. a developing station at which said layer of toner particles is presented in developing relation to an electrostatic latent image on an image-retaining member; and c. means adapted to maintain a potential difference between said screen electrodes and said film electrodes as each screen electrode region Is brought to said loading station, said potential being sufficient to create voltage induced microfields extending into the region beyond the surface of said donor member and of a magnitude sufficient to attract to and hold toner particles on the surface of said donor member carrying said screen electrodes, said means being further adapted, after each screen electrode region is loaded with toner, to effect a voltage induced field reversal of said microfields, said reversal being effective to remove charges from said dielectric layer accrued during the time period of toner loading, said means being further adapted, after charge removal, to sequentially return the microfields to their former direction.
 2. a developing station at which said layer of toner particles is presented in developing relation to an electrostatic latent image on an image-retaining member; and c. means adapted to maintain a potential difference between said screen electrode and each of said film electrodes as each film electrode region is brought to said loading station, said potential difference being sufficient to create voltage induced microfields extending into the region beyond the surface of said donor member and of a magnitude sufficient to attract to and hold toner particles on the surface of said donor member carrying said screen electrode, said means being further adapted, after each film electrode region is loaded with toner, to effect a voltage induced field reversal of said microfields, said reversal being effective to remove charges from said dielectric accrued during the time period of toner loading, said means being further adapted, after charge removal, to sequentially return the microfields to their former direction.
 3. An apparatus for developing an electrostatic latent image formed on the surface of an image-retaining member, said apparatus comprising: means for developing said latent image, said means including: a. a microfield donor member adapted to transport toner particles on the outer face thereof to said latent image comprising an endless dielectric layer sandwiched between and separating a continuous, electrically conductive, screen pattern electrode from a plurality of mutually electrically isolated conductive film electrodes with said screen electrode being on the outer face of said dielectric layer for attracting said toner particles to said donor member and said film electrodes being on the inner face of said dielectric layer; b. means to transport the surface of said donor member past a plurality of treating stations, said treating stations including:
 4. An apparatus for developing an electrostatic latent image formed on the surface of an image-retaining member, said apparatus comprising: means for developing said latent image, said means including: a. a microfield donor member adapted to transport toner particles on the outer face thereof to said latent image comprising an endless dielectric layer sandwiched between and separating a plurality of mutually electrically isolated conductive screen pattern electrodes from a plurality of mutually electrically isolated conductive film electrodes with said screen electrodes being on the outer face of said dielectric layer for attracting said toner particles to said donor member and said film electrodes being on the inner face of said dielectirc layer; b. means to transport the surface of said donor member past a plurality of treating stations, said treating stations including: 